1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and more particularly, to an image forming apparatus having large nozzles and small nozzles which eject liquid droplets of different volumes.
2. Description of the Related Art
An inkjet recording apparatus is known that is provided with large nozzles and small nozzles, which eject droplets of liquid or ink having mutually different volumes to be deposited onto a recording medium to form high-quality images having high tonal graduation. In a thermal jet method, which performs ejection by using heating elements, a composition including large nozzles and small nozzles is particularly beneficial, since it is difficult to achieve satisfactory control of the ejection of liquid droplets having different volumes from the same nozzle, in comparison with a piezoelectric method using piezoelectric elements as actuators.
Banding or non-uniformities may occur in the recorded image due to errors in the droplet ejection characteristics, such as the ejection volume, deposition position, or the like, of the liquid droplets ejected from the respective nozzles. As a method of reducing the visibility of banding and non-uniformities, Japanese Patent Application Publication No. 2004-148723, for example, discloses a method for arranging the dot pattern in such a manner that large and small dots formed by droplets ejected from large nozzles and small nozzles do not overlap with each other. Moreover, Japanese Patent Application Publication No. 2005-153435 discloses a method according to which large nozzles and small nozzles are alternatively arranged so that the intervals between large dots are covered over by small dots without leaving any spaces.
Furthermore, Japanese Patent Application Publication No. 2005-205718 discloses another method for reducing the visibility of banding and non-uniformities in an inkjet recording apparatus that performs recording by means of a so-called multi-pass method, in which the paper conveyance amounts for passes are changed in accordance with the error in the deposition positions of the liquid droplets ejected from the nozzles so that the combination of the nozzles is optimized to reduce the visibility of banding and non-uniformities.
However, the methods disclosed in Japanese Patent Application Publication Nos. 2004-148723 and 2005-153435 seek to reduce the visibility of banding and non-uniformities by appropriately adjusting the arrangement of the dot pattern or the nozzle arrangement, and they do not take any consideration of error in the droplet ejection characteristics of the large nozzles and the small nozzles. In general, in a composition including large nozzles and small nozzles, the large nozzles and the small nozzles do not display the same error tendencies in terms of their droplet ejection characteristics. Consequently, in these methods, there are limitations on the reduction of the visibility of banding and non-uniformities, and it is difficult to further improve the image quality.
In the method disclosed in Japanese Patent Application Publication No. 2005-205718, the fact that the paper conveyance amounts are not uniform is liable to place a burden on the paper conveyance system, and is likely to give rise to greater overall size of the inkjet recording apparatus, and increased costs. Moreover, this method is premised on an apparatus based on the multi-pass system, and it cannot be applied to a so-called single-pass system. Furthermore, using this method in a composition including large nozzles and small nozzles is problematic, since the large nozzles and the small nozzles do not have the same error tendencies in terms of the droplet ejection characteristics, as described above, and therefore it is not appropriate for images composed of large dots and small dots.